The present invention relates to apparatuses and methods for cellular communication. More specifically, the present invention relates to apparatuses and methods for intelligently cross-connecting bearer data paths between two cellular handsets at lower levels of a cellular network hierarchy.
In the traditional cellular communication system, such as one utilizing the Global Systems for Mobile Communication (GSM) protocol, the mobility management function (MM) is typically centralized at the Mobile-services Switching Center (MSC). It in turn provides a packet communications path between call control (CC), supplemental services (SS), short message services (SMS) and the mobile stations (MS units), also known as the cellular handsets. The call control (CC) function typically performs the call setup, which includes call switching, and the supplemental services (SS) function provides supplemental services to the MS units. As the name implies, the short message services (SMS) provides short message services to the MS units.
The traditional GSM network typically provides a public MSC for controlling a large geographical area. MS units within this geographical area connect to the public MSC for their mobility management, call control, supplemental services, and short message services needs.
In the prior art, the bearer data channel path that makes up part of the actual call path from each MS unit is backhauled all the way up to the public MSC. At the public MSC, it either enters the public wired network or is cross-connected with another bearer data channel path from another MS unit for the purpose of completing an end-to-end connection. This is because the public MSC of the prior art both has access to subscriber information and contains the actual call switching circuit for effecting the required cross-connection when an MS unit dials a telephone number associated with another MS unit. The functions of the call switching circuit includes, for example, routing based on the telephone number dialed.
To facilitate discussion, FIG. 1 shows in a simplified format a traditional GSM cellular communication network including public MSC 200, BSC's 202 and 204, and BTS's 206, 208, and 210. BSC 202 controls BTS 206, while BSC 204 controls BTS's 208 and 210. FIG. 1 also shows a plurality of MS units 220, 222, 224, 226, and 228. MS units 220 and 222 are controlled by BTS 206, MS units 224 and 226 by BTS 208, and MS unit 228 by BTS 210.
When MS unit 220 wants to build a call to MS unit 228 in the prior art, the mobility management session is built from MS unit 220 to public MSC 200 via BTS 206 and BSC 202. The destination phone number supplied by MS unit 220 enables public MSC 200 to locate MS 228 and then build a mobility management (MM) session to MS unit 228 via BSC 204 and BTS 210. Thereafter, the outgoing call path is built from MS unit 220 to public MSC 200 while the incoming call path is built from public MSC 200 to MS unit 228. The incoming and outgoing call paths are then cross-connected at public MSC 200 to complete the end-to-end connection.
When MS unit 220 desires to call MS unit 222 in the prior art, both the MM sessions and the call paths associated with these two MS units are built all the way to public MSC 200, where the connection cross-connect between the incoming and the outgoing paths occur. Likewise, when MS unit 224 wishes to call MS unit 228, both the MM sessions and the call paths associated with these two MS units are also built all the way to public MSC 200 to be cross-connected therein. In sum, the prior art requires that both the MM sessions and the call paths from MS units that participate in a call be built all the way to the public MSC in order for the call paths to be cross-connected, thereby completing the end-to-end connection. As is apparent from FIG. 1, neither of the above-discussed end-to-end connection via public MSC 200 represents the shortest route between the MS units participating in the call.
As the term is used herein, a cross-connect represents the creation of a data path across one node which allows data to flow from one end, e.g., from an input source, to another end, e.g. to an output source. The data path may optionally pass through resources. For example, the data path in a GSM implementation may pass through a TRAU. Further, the term end-to-end connection represents the connection of one phone to another, including all cross-connections within nodes that the end-to-end connection traverses as well as intervening facilities. A path, on the other hand, represents a piece of an end-to-end connection. Examples of paths include incoming telephone paths, outgoing telephone paths, upstream paths, and downstream paths.
As the term is used herein, a downstream path is defined from the perspective of the "current node" and refers to the piece of the path through the current node and all other nodes and facilities in the path from that node to the MS. Conversely, the upstream path refers to the piece of the path through the current node and all other nodes and facilities in the path from that node to the node which performs the cross connect between the incoming and outgoing call paths.
A path cross-connect refers to all the nodal cross-connects and the intervening facilities in the path. In contrast, a connection cross-connect refers to the cross-connection between the incoming call path and the outgoing call path across a single node.
The use of a public MSC to cross-connect bearer data circuits among MS units for the purpose of building calls has certain disadvantages. For example, the prior art public MSC, due to the prior art centralized switching approach, typically has a large domain and controls the cross-connecting of MS units in a wide geographical area. By way of example, it is not unheard of for an MSC to be located hundreds of miles from a BTS within its domain while bearing the sole responsibility for cross-connecting through it all calls involving MS units in its domain.
It has been recognized, however, that a large percentage, up to 50-75% in some cases, of calls made by an average user typically involves a destination MS unit located a short distance away from the caller. In some market, such as remote villages in developing countries or isolated communities or factories, for example, it has been determined that most calls are typically made to shops, homes, and facilities within a small radius of the calling unit, with a rather small percentage of calls being made over any appreciable distance (over 50 miles). Consequently, the prior art requirement of backhauling bearer data channels from MS units all the way back to a public MSC for cross-connection represents a wasteful use of the network bandwidth. This is especially true when an MS unit, which is located some distance away from the prior art public MSC, wishes to call a destination MS unit located nearby, which destination MS unit is also in the domain of the same public MSC. In this case, the prior art disadvantageously requires the bearer data paths to be backhauled all the way back to the central public MSC for cross-connection. Because of the prior art centralized switching approach, which centralizes the connection cross-connect function at the public MSC, the number of calls that can be handled by the prior art network is necessarily limited by the number of calls that can be cross-connected simultaneously by the prior art central switching public MSC and by the capacity of the facilities to that MSC.
Further, current implementations of MS units may send and receive data at different rates, say 8 Kbits, 16 Kbits or even 32 Kbits. As is well known to those of skill in the art, the bandwidth and voice or data encoding of MS units that communicate at different rates must be harmonized before their call paths can be cross-connected. Furthermore, current implementations of the public network, whether Public Switched Telephone Network (PSTN) or Public Land-based Mobile Network (PLMN), typically send and receive time division multiplexed data (TDM) at a fixed rate, say 64 Kbits. To accomplish end-to-end connection cross-connect, the prior art GSM system performs rate conversions (typically using a Transcoder Rate Adapter Unit, hence the name TRAU) on data from all MS units, whether or not they communicate at the same or different rates, by converting them to 64k bits. After the conversion, the 64 Kbits TDM data may be backhauled to the public MSC, using the network resources, for cross-connection therein.
It is known that TRAUing to perform rate conversions degrades communication quality and increases the network computational overhead. As mentioned before, TRAUing is performed in the prior art on data from and to MS units regardless whether they communicate at the same or different rates. It is recognized, however, that significant improvement on communication quality and optimizing of network computational resources may be achieved when unnecessary TRAUing, e.g. for calls between MS units that communicate at the same rate, can be eliminated.
Consequently, what is desired is a method and apparatus for reducing the amount of backhauling required in the prior art cellular communication system for cross-connecting call paths. In accordance with one aspect of the present invention, the inventive apparatus and method intelligently determines an optimum end-to-end connection and preferably accomplishes connection cross-connects of call paths at downstream nodes, i.e. lower levels in the network hierarchy, thereby reducing the distance by which data from MS units must be carried for completing the end-to-end cross-connection. In accordance with a further aspect of the present invention, the inventive apparatus and method preferably eliminates unnecessary usage of resources, such as TRAU, for the completion of end-to-end connections when those resources are not needed. When those resources are required, however, the inventive apparatus and method preferably switches them in as necessary at appropriate locations along the optimum end-to-end connection in order to properly complete the connection between MS units.